Improved tank collar

ABSTRACT

A tank collar assembly comprising: —(i) a first portion adapted at a first end to be connected to a tank around a manway or to a manway itself in a substantially fluid tight manner, said first portion being adapted to extend substantially entirely around the manway on the tank and to extend away from said tank or manway such that it forms an annular flange surrounding the manway; (ii) a second portion formed from a fusible material, said second portion being attached to the first portion in a substantially fluid-tight manner by a securing means; wherein a surface of said second portion is adapted to contact a corresponding fusible mating surface on the base of an access chamber.

FIELD OF THE INVENTION

The present invention relates to the construction of underground tanks. It is particularly applicable, but in no way limited, to collar assemblies used to form fluid tight connections between containment or access chambers or containment sumps and a tank. The invention includes methods of manufacturing such collars, to tanks incorporating them, and to chambers adapted to interface with the new tank collar assemblies.

BACKGROUND TO THE INVENTION

In typical underground storage and distribution systems for hazardous fluids such as hydrocarbon fuels, the fuels are usually stored in a large storage tank buried in the ground and delivered through underground piping to delivery pumps or the like. In order to ensure that the fuels cannot leak into the ground surrounding the tanks and pipework, so-called secondary containment systems are used which essentially provide a second barrier of protection around the primary fluid supply storage and delivery systems.

Typically, secondary containment systems have included containment sumps or access chambers, which are an offshoot from the so-called back fill retainer. There are a variety of chambers now on the market usually comprising a body defining an enlarged chamber, a riser connected to the body, where the riser is generally of smaller diameter than the body, and a cover fitting over the top end of the riser.

The containment or access chamber is installed below ground to provide a means of access to the manway, underground piping connections, submersible pumps, leak detection sensors, fire extinguisher and other plumbing components usually found connected to the top of underground storage tanks or under fuel dispensing units.

Access or containment chambers are multi-purpose in function:

-   -   1. They provide a means of surface access to equipment, plumbing         and miscellaneous devices installed underground.     -   2. They provide a means of ground isolation for contained         components to prevent corrosion and decay.     -   3. They provide a means of secondary containment for those         contained components which handle hazardous liquids.     -   4. They act as a collection sump for double wall piping entering         the access chamber.

Underground storage tanks usually have an access hatch or manway on the top of the tank to permit access into the interior of the tank if necessary. An access chamber is installed over the manway to facilitate access to the interior of the tank once it is underground. The access chamber is connected to the tank by means of a collar, which in turn is connected in a fluid tight manner to the outside of the tank around the manway. Where the tank is made of metal, a metal collar is normally welded to the tank. These collars are typically circular or polygonal in cross-section but can be of any suitable cross-section as determined by the designer, and to suit the particular manway/access chamber combination.

Protruding through and connected to the top of the manway are various pipes, elbows and connectors which are in turn connected to the underground pipework system, whose pipe ends usually enter through the side of the access chamber wall. This enables the fuel stored within the tank to be distributed to the pumps.

Due to the limited space within the chamber and the necessary size of the manway to permit a person to enter the tank, the connections between the pipes entering through the side of the chamber wall and the pipes/fittings protruding through the top of the manway are usually made directly over the manway itself. Typically, flanges are used to connect together the underground pipework to the pipes/fittings associated with the manway lid.

There are various methods by which an access chamber can be connected to the collar around the manway. In one method the collar incorporates an upwardly directed channel around its circumference, usually the outer circumference. The radius and profile of this channel is designed to accommodate a corresponding downwardly depending skirt on the base of the access chamber, the skirt having substantially the same cross-section as the collar channel. During installation the channel is filled with sealant, preferably a polyurethane sealant, before the chamber skirt is pushed into the channel. Once set, the sealant forms a substantially fluid tight seal between the chamber and the collar.

However, these sealants can break down over time and this can lead to ground water ingress into the chamber or escape of fuel from the chamber in the event of a fuel leak or spillage. Both these situations are serious and result in costly maintenance work being required.

Because this seal is buried in the ground, excavation is necessary to expose it, and the whole assembly generally has to be dismantled. Not only is this expensive in both money and time, but the garage or operating unit has to be closed to business while repairs are carried out. This results in significant lost revenue.

In another method, a collar is provided which incorporates a metal flange designed to engage with a corresponding flange on the bottom of the chamber. A gasket is provided between the two flanges during assembly and the metal flange is bolted to the bottom of the chamber. These gaskets degrade over time, resulting in similar consequences to those described above.

Modular access chambers made from plastics material are known, as for example in EP1,717,377 (NUPI S.p.A.). The modules of these modular chambers can be connected to each other by electrofusion means using special electrofusion beads. However, the bottom of such modular chambers is still connected to a tank collar by way of bolts through a conventional flange and a gasket. Electrofusing bonding tape or beads are known from GB2,407,795 (PetroTechnik Ltd).

It is an object of the present invention to provide a collar assembly for a tank which overcomes or mitigates some or all of the above disadvantages, and to provide tanks and access chambers incorporating improved collar assemblies.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a tank collar assembly according to claim 1. In one embodiment the tank collar assembly comprises:—

-   (i) a first portion adapted at a first end to be connected to a tank     around a manway or to a manway itself in a substantially fluid tight     manner, said first portion being adapted to extend substantially     entirely around the manway on the tank and to extend away from said     tank or manway such that it forms an annular flange surrounding the     manway; -   (ii) a second portion formed from a fusible material, said second     portion being attached to the first portion in a substantially     fluid-tight manner by a securing means;     wherein a surface of said second portion is adapted to contact a     corresponding fusible mating surface on the base of an access     chamber.

This arrangement provides, for the first time, the ability to electrofuse a chamber onto the tank collar.

Preferably the first portion is formed from a metal, especially when the tank is formed from metal.

Preferably said securing means comprising swaging, such that a substantially fluid tight swaged joint is formed between the radially extending flange of the first portion and the second portion. The technique of swaging is known per se and is a convenient method of forming a durable plastic to metal seal.

In an alternative preferred embodiment the second portion is secured to the first portion by moulding the second portion to, around or within the first portion.

In a further preferred embodiment the second portion is secured to the first portion by way of an adhesive or a sealant.

Preferably the second portion incorporates an energy transfer means adapted to heat a first surface of the second portion in order to form a substantially fluid tight seal with a mating surface of an access chamber, and more preferably the energy transfer means comprises one or more electrofusion heating elements.

Alternatively, the electrofusion heating element(s) may be incorporated into the mating surface of the access chamber.

Preferably the second portion rests on and is supported by the first portion in use.

Preferably the assembly further comprises one or more support means, said support means being adapted to support the first portion. Struts, stays or brackets may be used to strengthen the flange.

Preferably the assembly is connected to the tank, wherein the first portion comprises two components, a first component connected in a substantially fluid-tight manner to the tank around the manway, and a second component is connected to the first component in a substantially fluid-tight manner, said second component extending substantially entirely around the first component in the form of a radially extending flange, the first and second components being joined during manufacture.

Preferably the first component and the second component are both made of a metal and preferably the two components are welded together.

According to a second aspect of the invention there is provided a tank incorporating a tank collar assembly as claimed herein.

According to a third aspect of the invention there is provided an access chamber incorporating a fusible surface in its base said fusible surface being adapted to mate with a corresponding fusible surface in a tank collar assembly as claimed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example only with reference to the accompanying drawings, wherein:

FIG. 1 is a partially cut-away side view of part of a petroleum forecourt installation, which includes a tank having a containment chamber, and a pair of dispensing pumps having sump chambers;

FIG. 2 illustrates a side view of a prior art containment chamber assembly together with a storage tank;

FIG. 3 illustrates a cut-away perspective view of the containment chamber in FIG. 2;

FIG. 4 illustrates a cross-sectional view of the prior art containment chamber of FIG. 2;

FIG. 5 illustrates a cross-sectional view of a first embodiment of a collar assembly according to the present invention attached to a tank and in combination with an access chamber;

FIG. 6 illustrates a flange component of the collar assembly in FIG. 5;

FIG. 7 illustrates a cross-section A-A of FIG. 6;

FIG. 8 illustrates a cross-sectional view of a second embodiment of a collar assembly according to the present invention.

FIG. 9 shows a tank collar according to a further embodiment of the invention with an access chamber attached;

FIGS. 10 and 11 show enlargement of Detail A and Detail B of FIG. 9 respectively;

FIG. 12 illustrates in exploded diagram form the assembly of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present embodiments represent currently the best ways known to the applicant of putting the invention into practice. But they are not the only ways in which this can be achieved. They are illustrated, and they will now be described, by way of example only. By way of terminology used in this document the following definitions apply:—

Access chamber—any receptacle designed to keep a fluid in or out. This includes, but is not limited to, access, manhole and sump chambers as described herein. It also includes tanks in general. Access chamber system—any part of the underground system, including the access chamber, that is contained by, or attached to the access chamber. This includes the access chamber itself, corbel, frame neck or lid together with the underground tank, collar, manway and associated pipework. Energy transfer means—a generic term describing any form of energy source. Typically it takes the form of a resistance winding which heats up when an electrical current is passed through it. The term also encompasses other welding techniques including ultrasonic welding and induction welding. Flange—any collar suitable for attaching the tank collar assembly to an access chamber. In the examples given the surface of the flanges are substantially planar. However, it will be understood that the flange must conform to the profile of the section to which it is to be joined. Thus the flange can adopt any suitable configuration or conformation to achieve the necessary contact with a flat or curved surface. Fluid—whilst the examples provided relate mainly to liquids, the term fluid refers to liquids, vapours and gases. For example, should a leak occur in a secondarily contained pipe in a garage forecourt installation then petrol or petrol vapour will collect in the access chamber. It is essential that this petrol vapour cannot escape through the tank collar assembly and into the surrounding ground. Pipe—where pipes are referred to herein they are generally of circular cross-section. However, the term also covers other cross-sections such as box sections, corrugated and the like and secondarily contained pipes of the “pipe-within-a-pipe” type. Glass reinforced plastic (GRP)—The term GRP has a very broad meaning in this context. It is intended to encompass any fibre-reinforced plastic wherein a fibre of any type is used to strengthen a thermosetting resin or other plastics material. Fusible material—The term fusible material has a very broad meaning in this context. It is intended to encompass any polymeric material which when energy is applied to it can melt and fuse together with an adjacent material and is intended to cover thermoplastics, thermosets, elastomers and adhesives. Plastics Material—The term has a very broad meaning in this context and is intended to encompass any polymeric material including thermoplastics, thermosets, elastomeric or any other polymeric material.

The petroleum forecourt installation shown in FIG. 1 comprises a pair of dispensing pumps 10 and 11 connected to a subterranean tank 12 through a pipeline 13. The pipeline 13 is formed from contiguously arranged sections of polyethylene pipe. The pipeline 13 extends from the pumps 10 and 11 through the sump chambers 14 and 15 having side walls 16 and bases 17 into a containment or access chamber 18 having a side wall 19 and a base 1 immediately above the tank 12.

FIG. 1 shows two lines extending from the pipeline 13 into the tank 12. These lines relate to two alternative forms of fuel supply system and are both shown for the sake of completeness. In practice, only one of the lines would extend from the pipeline 13 into the access chamber 18. One of those lines is a suction line 2 which is used where the dispensing pumps 10 and 11 are fitted with suction pumps. The alternative line, reference 3, is a pressure line connected to the pipeline 13 via a pump 4 which is operable to propel fuel from the tank 12 to the pumps 10 and 11.

It can be seen from FIG. 1 that the walls 16 and 19 have to be apertured in order to allow the pipeline 13 to pass into the chambers. In order to prevent water leaking from the surrounding ground (here denoted by reference numeral 5) into the chambers 14, 15 and 18 through the aperture, the pipe is sealed to the walls 16 and 19 by means of a fitting. In the event of a spillage or a leak in a supply pipe the seal also prevents fuel from escaping into the environment.

FIGS. 2 to 4 show various views of a prior art containment or access chamber mounted on a tank. The chamber arrangement can be seen more clearly in FIG. 3, which shows a cut-away view of the chamber in place on a storage tank. It can be seen that the incoming pipework and its associated fittings are outside the vertical footprint of the manway and that the lower aperture and the upper aperture are concentric. This can be seen by line 51 in FIG. 4. Detail B shows a detailed view of a channel that contains a polyurethane sealant 50. The sealant is used to form a fluid tight seal between the chamber and the upstanding collar and holds the chamber in place.

FIG. 3 shows a cut away perspective view of a containment chamber and shows the upper aperture 52, access cover 53, access cover skirt 54, chamber corbel 55, chamber body 56 and chamber base 57. It also shows manway 58 and incoming pipework 59, together with tank 60.

The prior art containment chamber of FIGS. 2 to 4 is a decagonal (10 sided) shape, which means that it is extremely rigid and is compatible with all entry fitting design criteria. The asymmetrical offset design means that the chamber has one large side for the 90 mm-110 mm secondary contained fill lines. Therefore, the 4-inch fill line flange fittings can be located outside the manway lid footprint allowing easy removal of the tank manway lid when required.

Underground storage tanks, particularly those used for storing petroleum products, are generally made of metal. The tank collars associated with such tanks are also usually made of metal, and welded to the top of the tank around a manway. Access chambers on the other hand are usually made from a plastics material such as polyethylene. This is much lighter and more durable than metal, easier to form into the desired shape, and is compatible with the polyethylene pipework widely used to convey petroleum products. This therefore presents the problem of how to bond a plastic chamber to a metal tank collar in a substantially permanent and substantially fluid-tight manner, in a way which improves on the methods already known in the art.

The direct sealing method shown in detail B of FIG. 4 means that the chamber base fits into a groove on the tank collar filled with GRP (or some other) resin and the two surfaces bond to become one. If required, the chamber base can be bonded onto the tank collar in the factory before delivery, dramatically reducing site installation time.

It should be noted that the term “access chamber” is analogous to the term “containment chamber” and both terms can be used interchangeably. The present invention provides an improved tank collar assembly and an improved sealing method for sealing the base of an access chamber to a tank collar assembly.

The access chamber assembly can be manufactured from a variety of materials as selected by the materials specialist. Preferably the base section and the riser section are formed from the same material. By way of examples only, suitable plastics materials may be selected from the group comprising:—

-   -   polyethylene;     -   polypropylene;     -   polyvinyl chloride;     -   polybutylene     -   polyurethanes;     -   polyamides, including polyamides 6, 6.6, 6.10, 6.12, 11 and 12;     -   polyethylene terphthalate;     -   polybutylene terephthalate;     -   polyphenylene sulphide;     -   polyoxymethylene (acetal);     -   ethylene/vinyl alcohol copolymers;     -   polyvinylidene fluoride (PVDF) and copolymers;     -   polyvinyl fluoride (PVF);     -   tetrafluoroethylene-ethylene copolymer (ETFE);     -   tetrafluoroethylene-hexafluroethylene copolymers (FEP)     -   ethylene tetrafluoroethylene hexafluropropylene terpolymers         (EFEP)     -   terpolymers of tetrafluoroethylene, hexafluoropropylene and         vinylidene     -   fluoride (THV);     -   polyhexafluoropropylene;     -   polytetrafluoroethylene (PTFE);     -   polychlorotrifluoroethylene;     -   polychlorotrifluoroethylene (PCTFE);     -   fluorinated polyethylene;     -   fluorinated polypropylene;         and blends and co-polymers thereof. Furthermore, it is known to         use blends of two or more polymers and this invention extends to         cover known and yet to be developed blends of plastics material.         The plastics used to construct access chambers are generally         electrofusible.

An improved tank collar assembly according to a first embodiment of the present invention is illustrated in FIGS. 5 and 6. FIG. 5 shows a tank 70 which incorporates a manway 68. Encircling the manway is a first portion 69,71 of the tank collar assembly. A first component of this first portion comprises an upstanding collar 69 which extends substantially entirely around the manway 68 and is spaced radially away from it. In this example the tank is made of steel and the lowermost-in-use edge of this first component is welded to the tank to form a substantially permanent, substantially fluid tight seal to prevent ingress of water or egress of fuel in the event of a leak or spillage.

Instead of carrying a channel around its uppermost in use edge, the first component 69 is attached to a second component 71. This second component 71 takes the form of an annular ring or flange extending radially away from the first component and secured to the outside of the first component around the inner radius of the second component in a substantially fluid tight manner. In this example the second component is also made from steel and the first and second components are welded together.

Whilst in this example the first and second components are shown as separate components, welded together to become an integral unit, it is perfectly possible that the first and second components could be formed by or during manufacture as an integral unit.

The outer region of the annular flange 71 is attached to a second portion 73 in a substantially fluid tight manner by securing means, the second portion being formed from a fusible material. In this example the securing means is a swaged joint 74.

Some suitable fusible plastics materials are listed above. Chambers are often made from polyethylene and in this example the annular flange 71 is also made of polyethylene (see below).

Provision is made in the outwardly projecting radially extending annular flange 71 for electrofusion heating element(s) 75. These heating element(s) are located on the uppermost-in-use face of flange 71 and are used to fuse the top of the annular flange 71 to the bottom of a correspondingly sized and shaped annular flange 67 on the base of chamber 65. The heating element(s) may take the form of a wire or wires embedded in the surface of the flange, with the ends of the wire(s) being connected to electric terminals (not shown). This arrangement of heating wires 75 is shown in more detail in FIG. 7, together with the swaged joint between the first and second portions.

In an alternative embodiment the electrofusion heating elements may take the form of a pre-formed tape or a pre-formed core of heating elements. An indentation or channel can be formed to accommodate such a tape or core and the tape or core can be secured in such a channel during manufacture, such that the tank collar arrives at site with the electrofusion tape or core already in position for the electrofusion process. For example, the tape/core may be tack welded in place.

Plastic chambers are traditionally formed by rotomolding and are often formed from Linear Low Density Polyethylene (LLDPE). As such it is therefore preferable that the fusible flange on the tank collar assembly is formed from a Polyethylene of similar density. Alternatively, the flange could be formed from Medium Density Polyethylene (MDPE) or from High Density Polyethylene (HDPE). It has unexpectedly been discovered that all three grades of Polyethylene form adequate electrofusion bonds with chambers made from LLDPE, and vice versa.

If a different plastics material is used for the chamber, such as Polypropylene, then a compatible polymer is used in the fusible flange collar. A materials specialist will select a suitable compatible plastics material for this use.

In summary, this embodiment of a tank collar assembly comprises an annular collar, flange or disc made from a fusible material and attached by a securing means in a substantially fluid tight manner to an upstanding collar adapted to be attached to a tank around a manway. This arrangement provides, for the first time, the ability to electrofuse the base of an access chamber, formed from a fusible plastics material such as polyethylene, to a tank collar. There may, or may not, be some intermediate portion between the upstanding collar and the flange or disc.

A second embodiment is illustrated in FIG. 8. In this embodiment the annular disc or flange of fusible material 93 is supported by and accommodated substantially on the annular metal flange ring 91 extending away from the upstanding collar 89 or manway. In this embodiment the annular ring 91 is also made of steel and is spun swaged at one edge, the edge furthest from the manway, over the fusible disc 93.

To add strength and support to the assembly, support means in the form of brackets 96 support the steel plate 91 as it extends from the upright member or portion 89.

In an important feature of this invention, and because of the flexibility of this invention, it is possible to dispense with the separate upstanding collar normally attached to the top of the tank around the manway. This can be achieved by welding the radially extending plate or flange 91 directly to the outside of the manway. This limits the amount of construction needed on site, thus reducing cost, installation time and materials. The radially extending plate or flange 91 can extend away from and around the manway cover for a significant distance, sufficient to give the desired amount of room within the tank chamber or sump both for the required connections and to allow an operator access. The support means 96 can be of a size, design and extent to give sufficient support to the flange 91 and to the tank sump 87 until the area under the sump has been back filled.

A further embodiment is shown in FIGS. 9 to 12 inclusive. These illustrate a tank 190 with a manway 158 around which is formed a tank collar 169. The detail of the flange and sealing arrangement for sealing an access chamber body 156 to the flange is shown in Detail A shown in FIG. 10. This illustrates a circumferential flange 171 as an integral part of a tank collar ring 169. Held captive within the flange by a swaged joint is a polyethylene ring 174. this ring includes a series of heating elements 175, set into a matrix of polyethylene or some electrofusibly-compatible plastics material. This is housed in a shallow channel set into the polyethylene ring. This construction provides a PE tank transition collar with an integrated welding coil.

The base 157 of an access chamber body 156 is placed onto the ring and current passed through the heating elements in a known fashion to electrofuse the polyethylene ring to the chamber base in a fluid-tight fashion. These components are shown in exploded diagram format in FIG. 12.

A similar integrated electrofusion heating coil is provided in the bottom of chamber riser.

This second integrated electrofusion coil enables the chamber riser to be electrofusion welded to the chamber base 156.

Whilst the electrofusible ring on the tank collar is shown as being substantially horizontal, when the tank collar is in its usual installed configuration, it will be appreciated that this is not essential. For example, the flange of the tank collar could be angled inward somewhat, at an angle of between 1° and 20° such that the chamber base will self-centre once placed onto the flange.

It will be appreciated that the materials used in each of the components or portions will be decided by materials specialist as appropriate, according to the application and to material from which the tank is constructed. Where the tank is made say from GRP, then a first portion, preferably made of metal, is joined in a substantially fluid-tight manner to the GRP tank. This could be by way of a swaged connection incorporating an internal sealing means, such as a rubber ring or gasket, as necessary. A second portion made from an electrofusible plastics material is then joined to the first portion in a substantially fluid-tight manner, again possibly by swaging. Or alternatively the plastics material could be moulded around or into the first portion with a fluid-tight seal therebetween.

In summary, an equivalent form of construction to that described elsewhere in this document is possible for a GRP tank, as well as for a metal tank.

KEY TO FIGURES FIG. 5

-   70 Tank

FIG. 7

-   71 Steel ring -   73 PE disc -   74 Spun swage -   75 Electro fusion coil

FIG. 8

-   85 Tank sump -   90 Tank -   91 Steel plate -   93 PE ring -   94 Spun swage -   95 Electrofusion core

FIG. 9

-   190 Deep burial chamber

FIG. 12

-   153 PE water tight chamber lid -   154 PE chamber riser with electrofusion coil—welds to chamber base -   156 PE chamber base—welds to tank transition collar -   158 Manway -   169 Steel collar welded to tank -   190 Storage tank 

1. A tank collar assembly comprising:— (i) a first portion adapted at a first end to be connected to a tank around a manway or to a manway itself in a substantially fluid tight manner, said first portion being adapted to extend substantially entirely around the manway on the tank and to extend away from said tank or manway such that it forms an annular flange surrounding the manway; (ii) a second portion formed from a fusible material, said second portion being attached to the first portion in a substantially fluid-tight manner by a securing means; wherein a surface of said second portion is adapted to contact a corresponding fusible mating surface on the base of an access chamber.
 2. A tank collar assembly as claimed in claim 1 wherein the first portion is formed from a metal.
 3. A tank collar assembly as claimed in claim 2 wherein said securing means comprising swaging, such that a substantially fluid tight swaged joint is formed between the radially extending flange of the first portion and the second portion.
 4. A tank collar assembly as claimed in claim 1 wherein the second portion is secured to the first portion by moulding the second portion to, around or within the first portion.
 5. A tank collar assembly as claimed in claim 1 wherein the second portion is secured to the first portion by way of an adhesive or a sealant.
 6. A tank collar assembly according to claim 1 wherein the second portion incorporates an energy transfer means adapted to heat a first surface of the second portion in order to form a substantially fluid tight seal with a mating surface of an access chamber.
 7. A tank collar assembly as claimed in claim 6 wherein said energy transfer means comprises one or more electrofusion heating elements.
 8. A tank collar assembly as claimed in claim 7 wherein said electrofusion heating element(s) are incorporated into the mating surface of the access chamber.
 9. A tank collar assembly as claimed in claim 6 wherein the second portion rests on and is supported by the first portion in use.
 10. A tank collar assembly as claimed in claim 1 wherein the assembly further comprises one or more support means, said support means being adapted to support the first portion.
 11. A tank collar assembly as claimed in claim 1 and wherein the assembly is connected to the tank, wherein the first portion comprises two components, a first component connected in a substantially fluid-tight manner to the tank around the manway, and a second component is connected to the first component in a substantially fluid-tight manner, said second component extending substantially entirely around the first component in the form of a radially extending flange, the first and second components being joined during manufacture.
 12. A tank collar assembly as claimed in claim 11 wherein the first component and the second component are both made of a metal.
 13. A tank collar assembly as claimed in claim 12 wherein the two components are welded together.
 14. (canceled)
 15. A tank incorporating a tank collar assembly as claimed in claim
 1. 16. An access chamber incorporating a fusible surface in its base said fusible surface being adapted to mate with a corresponding fusible surface in a tank collar assembly according to claim
 1. 